The present invention relates to an FSK laser transmitting apparatus.
Since an output light intensity can be easily modulated upon on/off operation of injection current in semiconductor lasers, they have been used as light sources of optical fiber communications in a variety of applications. Furthermore, an oscillation frequency of a semiconductor laser varies due to a small variation in injection current, so that signal transmission can be performed in accordance with a so-called frequency shift keying (FSK) scheme. By utilizing FSK signal transmission with heterodyne detection, an effective communication system is being developed to achieve long-distance transmission with a good light reception sensitivity effect.
A frequency difference between symbols of the signal must be given as a predetermined value in the FSK system. However, dependency of an oscillation frequency or oscillation wavelength of the semiconductor laser on the modulation current is not constant. In addition, the oscillation frequency or wavelength also greatly varies in accordance with the structure parameters of the semiconductor laser, the bias current and the modulation frequency.
As is known well, for example, the oscillation frequency varies in the range of 100 MHz to several GHz per 1-mA modulation current. For this reason, frequency modulation in the semiconductor laser must be performed such that a frequency shift in the signal transmitter is given as a predetermined value. However, a conventional signal transmitter consists of a light source and its driver. Therefore, no control for frequency shift is provided in any conventional system.